IB Biology HLCellular RespirationPaper 1 & 2~11 min read
Respiratory Substrates
Everything so far has assumed glucose. But a hibernating bear and a germinating sunflower seed are both running mostly on fat — and fat carries more than twice the energy per gram. The reason comes down to counting oxygen atoms.
📚 What you need to know
Lipids transfer more than twice the energy per gram as carbohydrates when oxidised.
The reason: lipids have fewer oxygen atoms per molecule, which makes their hydrogen and carbon atoms more oxidisable.
Lipids are good storage molecules because they are insoluble, so they do not lower the water potential of the cell.
If they did lower it, water would move into the cell from nearby cells by osmosis.
Fat stores help animals survive unfavourable conditions; plants store oil reserves in seeds and fruits.
Oxidising lipids produces much more metabolic water than the same mass of carbohydrate — useful for desert animals.
Glycolysis and anaerobic respiration can only occur if carbohydrate is the substrate.
Lipids must first be broken into fatty acids, then into 2C acetyl groups, which join coenzyme A to form acetyl CoA and enter the Krebs cycle.
Carbohydrates are hydrolysed more easily, so their energy is transferred more quickly.
Energy per gram
An animal storing energy as fat rather than carbohydrate carries less than half the weight around — which matters a great deal if it has to fly or migrate.
Why lipids hold more
Respiration releases energy by oxidising the fuel. A molecule that is already partly oxidised has less left to give.
Carbohydrates contain plenty of oxygen atoms — think of the formula of glucose — so a good share of their carbon and hydrogen is already bonded to oxygen. Lipids contain far fewer oxygen atoms per molecule, so their hydrogen and carbon atoms are more oxidisable. There is simply more oxidising left to do, and every oxidation releases energy.
A useful way to picture it: a carbohydrate has already been through part of the process, so you are buying a partly used battery. A lipid is a fresh one. Same chemistry, different starting point.
Why lipids make good stores
They are insoluble, so they do not affect the water potential of the cell. A soluble store would lower the water potential and pull water in from nearby cells by osmosis, swelling the cell.
They are compact for the energy they hold, so animals can build fat stores to survive periods of cold or food shortage.
Plants use the same solution, storing oil reserves in seeds and fruits — a seed needs a dense energy supply it can carry.
Oxidising lipids produces a large volume of metabolic water. Some desert animals drink very little and rely heavily on this.
Why we still use carbohydrate first. Lipids are the better store, but carbohydrates are hydrolysed more easily, so their energy is transferred more quickly. Glucose is also the only substrate that can enter glycolysis directly — which is why a sprinter relies on carbohydrate and a migrating bird relies on fat.
How each substrate enters the pathway
This is the point most students miss. Only carbohydrate can be used for glycolysis, and therefore only carbohydrate can support anaerobic respiration. Lipids and proteins have to be converted into something the pathway already recognises, and they join it further along.
Trace the lipid route with your finger: it never touches glycolysis. Without oxygen, the Krebs cycle cannot run, so fat cannot be used at all.
Comparing lipids and carbohydrates
Feature
Lipids
Carbohydrates
Energy content per gram
Higher — more than twice as much
Lower
Metabolic water on oxidation
Produces a higher volume
Produces a lower volume
Solubility in cells
Insoluble, so osmotic properties are unaffected
Soluble, so osmotic properties are affected
Ease of breakdown
Hydrolysed less easily, so energy is transferred more slowly
Hydrolysed more easily, so energy is transferred more quickly
Can enter glycolysis?
No — must be converted to acetyl CoA
Yes, directly
Usable anaerobically?
No
Yes
Worked examples
WE 1
Explain the energy difference
Explain why the oxidation of lipids releases more energy per gram than the oxidation of carbohydrates. (3 marks)
Point 1: the structural difference
Lipids contain fewer oxygen atoms per molecule than carbohydrates do.
Point 2: what that means chemically
Their hydrogen and carbon atoms are therefore more oxidisable — less of the molecule has already been oxidised.
Point 3: the consequence
More oxidation reactions can occur per gram, releasing more hydrogen for the electron carriers, so more energy is transferred to ATP.
Fewer oxygens means more left to oxidise“more oxidisable” is the phrase mark schemes reward; avoid vague answers like “fat has more energy”
WE 2
Compare two stores by mass
Lipids release about 37 kJ g−1 and carbohydrates about 17 kJ g−1. Calculate the energy from 10 g of each, the difference, and how many times more energy the lipid store provides. (3 marks)
Step 1: lipid
10 × 37 = 370 kJStep 2: carbohydrate
10 × 17 = 170 kJStep 3: compare
Difference = 370 − 170 = 200 kJ. Ratio = 370 ÷ 170 = 2.2 (to 2 s.f.)
370 kJ and 170 kJ — a difference of 200 kJ, or about 2.2 times as much“more than twice” matches the standard statement, so it is a good sense-check
WE 3
Apply it to a desert animal
Suggest why storing energy as lipid rather than carbohydrate is an advantage for a desert mammal. (3 marks)
Point 1: water
Oxidising lipid produces a much larger volume of metabolic water than the same mass of carbohydrate, which matters where drinking water is scarce.
Point 2: mass
Lipids hold more than twice the energy per gram, so the same energy reserve weighs less to carry.
Point 3: osmosis
Lipids are insoluble, so a large store does not lower the water potential of the cells or draw water in by osmosis.
More water, less weight, no osmotic problemthe osmosis point is the one most students leave out
💡 Exam tips
Explain the lipid energy advantage using fewer oxygen atoms and more oxidisable.
Remember only carbohydrate can be used in glycolysis and therefore in anaerobic respiration.
Learn the lipid entry route: lipid → fatty acids → 2C acetyl groups → acetyl CoA → Krebs cycle.
Use metabolic water as the term for water produced by respiration.
Link insolubility to water potential and osmosis, not just “it does not dissolve”.
Note that carbohydrates release energy faster even though lipids release more.
⚠ Common mistakes
Saying lipids can be respired anaerobically. They enter after glycolysis, so they cannot.
Saying lipids enter glycolysis. They are converted to acetyl CoA and join at the Krebs cycle.
Saying lipids contain no oxygen. They contain fewer oxygen atoms, not none.
Treating proteins as a normal fuel. They are used only when glucose and lipids are unavailable.
Confusing “more energy” with “faster energy”. Lipids give more; carbohydrates give it quicker.
Forgetting metabolic water when asked about desert or hibernating animals.
That completes Cellular Respiration — glucose is oxidised in small steps, the hydrogens are collected by NAD and FAD, and oxygen pulls them down the chain so their energy can be captured as ATP. Up next: The Process of Photosynthesis, which runs the same machinery in reverse to build the glucose in the first place.
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